Mullard High Speed Valve Tester problem - UK Vintage Radio Repair and Restoration Discussion Forum

brymac · 18th Apr 2007, 1:07 pm

I'v bought a Mullard tester and full set (two boxes) of cards on eBay for £108 - the tester is in very clean condition, and comes with all the valve base adaptors and the manual.

I've started by going through the test cards - whilst the Reject and HT cards give the expected results once calibrated using the pots in the drop down panel, I can't get the mains check card to shift the spot down to the dotted line, despite moving the mains fine adjustment knob. The coarse mains appears to be set at 240V - I've changed it to 220V but with no result.

I've not taken it apart yet, so don't know the state of the caps. Before I do, have I missed something??

Bryan

Colin · 18th Apr 2007, 1:33 pm

Hi Bryan,

Have you tried setting the mains adjustment first? I seem to remember that all the adjustment pots have an effect on each other and if you set the mains adjust first (you may have to re-adjust the Reject and HT settings to get it right) then move on to the other settings. It was an iterative process on mine and it took a few loops around the cards to get all three settings calibrated correctly.

Rgds
Colin

brymac · 18th Apr 2007, 2:16 pm

Colin - thanks. I can get the mains adjustment right if I move the reject settings (there are no HT settings), but when I reset the reject settings (knobs "B" & "C"), the mains goes off again...... It doesn't seem possible to get all three settings right....... Bryan

wireless_paul · 18th Apr 2007, 4:17 pm

Quote:

Originally Posted by brymac

Colin - thanks. I can get the mains adjustment right if I move the reject settings (there are no HT settings), but when I reset the reject settings (knobs "B" & "C"), the mains goes off again...... It doesn't seem possible to get all three settings right....... Bryan

Paul Stenning has worked out how to adjust Preset Control "A" and has a sheet explaining how to do this. Leaky capacitors can also cause a problem.
Paul

GMB · 18th Apr 2007, 5:13 pm

The MAINS card is the first one to use. If it doesn't give a sensible result then I suggest you check the capacitors as if they are original then it is near certain that they will need to be replaced.

brymac · 18th Apr 2007, 5:59 pm

Thanks - I'm just working through all the caps and replacing them - have also found a few high value resistors. How do I obtain Paul Stenning's sheet about the dreaded "do not touch under any circumstances" knob "A" please?

Bryan

jim_beacon · 18th Apr 2007, 7:01 pm

Bryan,

the mains check card uses a resistor (mounted on the gate contact frame), which is not used for any other measurement. If you have the manual, you'll be able to identify it. I would check it, and its contacts.

Jim.

brymac · 18th Apr 2007, 7:08 pm

Jim - thanks; will do..... Bryan

PJL · 18th Apr 2007, 9:44 pm

I did a full restoration on one of these about a year ago and about 50% of the resistors were out of tolerance (mainly the low tolerance ones). Other problems are the valves as it is easy to leave the power on and hence destroy the valve emissions. Don't test it on low mains settings as there are some quite expensive valves in there.

The MAINS card only uses the CRT differential amplifier, the -ve supply and reference voltage so it is best to get this working first. Then move onto the REJECT card and finally the HT card which will fire up the regulated ht supply.

The preset A is used to calibrate the MAINS card but you should only adjust this if you are confident that the rest of the tester is properly to tolerance. Accurate mains settings are required for correct heater voltages as the reference and HT are regulated (i.e. using a DVM and setting it manually is fine).

I have produced a computer based calculator so if you PM me a card number (preferably a pentode) I can send you the voltages to expect on each pin of the test socket before you go melting a valve...

PJL · 18th Apr 2007, 10:20 pm

Here is the calculators output for card 68 for an EF80. You can expect some variation due to:
- Tolerancing errors in your tester including the reference voltage.
- The anode voltage is based on a good valve which is deemed to be one whose deflection in position 6 reaches the dotted line mark. This approximation is far from true but the impact on the results is slight as the anode load is generally small.
- The screen grid current is assumed to be 10% if I remember correctly but again it has only a minor impact on the voltage for some valves (not in this case).

brymac · 19th Apr 2007, 12:09 am

Thank you - that is very helpful. I'll have replaced all the capacitors by the weekend and will report progress then! Bryan

brymac · 19th Apr 2007, 11:54 pm

Quote:

Originally Posted by jim_beacon

Bryan,

the mains check card uses a resistor (mounted on the gate contact frame), which is not used for any other measurement. If you have the manual, you'll be able to identify it. I would check it, and its contacts.

Jim.

Found it - but it was OK!! Thanks anyway - I've got two caps arriving in the post tomorrow and then I'll have replaced them all - I shall then use Paul's sheet to attack the "do not touch" red knob "A"!!

PJL · 20th Apr 2007, 12:40 pm

Hi Bryan, I would not recommend following Paul's instructions just yet. This is how I would suggest you go:
- Measure your mains with a DMM and set the mains selector accordingly.
- With the MAINS card in check the -ve supply (-200V) and regulator (-85V) with a DMM. Don't worry about the spot position as the CRT amp has not been calibrated yet.
- With the REJECT card you can check proper operation and calibrate the CRT differential amplifier by following the Mullard instructions. It takes a while getting this right as the controls interact.
- With the HT card you can check the HT regulator is working (off load).
- With card 68 and the information I have supplied above you can check much of the rest of the circuitry: position 1 - pin 5 or 6 should read -200V; position 2 - pin 5-6=6.3V AC and pressing buttons should apply -126V...; position 6: pin 2=-2V, pin 7=170V...
- Once it is all running then calibrate the MAINS card using Pauls instructions

brymac · 20th Apr 2007, 12:46 pm

Many thanks - I'll try that! The new caps have just arrived, so I'll fit them first. One thing I have noticed is that at the moment the mains fine adjuster makes no difference to the spot position...

Bryan

PJL · 20th Apr 2007, 1:16 pm

Could be quite a few things - the negative supply is not working - the matrix contact is not making - the differential amplifier is not working - the function switch needs cleaning...if you get stuck I know the circuit pretty well so just put a message here.

18th Apr 2007, 1:07 pm	#1
brymac Hexode Join Date: Nov 2004 Location: East Sussex, UK. Posts: 389	Mullard High Speed Valve Tester problem I'v bought a Mullard tester and full set (two boxes) of cards on eBay for £108 - the tester is in very clean condition, and comes with all the valve base adaptors and the manual. I've started by going through the test cards - whilst the Reject and HT cards give the expected results once calibrated using the pots in the drop down panel, I can't get the mains check card to shift the spot down to the dotted line, despite moving the mains fine adjustment knob. The coarse mains appears to be set at 240V - I've changed it to 220V but with no result. I've not taken it apart yet, so don't know the state of the caps. Before I do, have I missed something?? Bryan

18th Apr 2007, 1:33 pm	#2
Colin Heptode Join Date: Jun 2004 Location: Ilkley, West Yorkshire, UK. Posts: 656	Re: Mullard High Speed Valve Tester problem Hi Bryan, Have you tried setting the mains adjustment first? I seem to remember that all the adjustment pots have an effect on each other and if you set the mains adjust first (you may have to re-adjust the Reject and HT settings to get it right) then move on to the other settings. It was an iterative process on mine and it took a few loops around the cards to get all three settings calibrated correctly. Rgds Colin __________________ Colin Armstrong

18th Apr 2007, 2:16 pm	#3
brymac Hexode Join Date: Nov 2004 Location: East Sussex, UK. Posts: 389	Re: Mullard High Speed Valve Tester problem Colin - thanks. I can get the mains adjustment right if I move the reject settings (there are no HT settings), but when I reset the reject settings (knobs "B" & "C"), the mains goes off again...... It doesn't seem possible to get all three settings right....... Bryan

18th Apr 2007, 5:13 pm	#5
GMB Dekatron Join Date: Aug 2003 Location: near Reading (and sometimes Torquay) Posts: 3,099	Re: Mullard High Speed Valve Tester problem The MAINS card is the first one to use. If it doesn't give a sensible result then I suggest you check the capacitors as if they are original then it is near certain that they will need to be replaced.

18th Apr 2007, 5:59 pm	#6
brymac Hexode Join Date: Nov 2004 Location: East Sussex, UK. Posts: 389	Re: Mullard High Speed Valve Tester problem Thanks - I'm just working through all the caps and replacing them - have also found a few high value resistors. How do I obtain Paul Stenning's sheet about the dreaded "do not touch under any circumstances" knob "A" please? Bryan

18th Apr 2007, 7:01 pm	#7
jim_beacon Retired Dormant Member Join Date: Mar 2003 Location: Bishop's Waltham, Hants, UK. Posts: 939	Re: Mullard High Speed Valve Tester problem Bryan, the mains check card uses a resistor (mounted on the gate contact frame), which is not used for any other measurement. If you have the manual, you'll be able to identify it. I would check it, and its contacts. Jim.

18th Apr 2007, 7:08 pm	#8
brymac Hexode Join Date: Nov 2004 Location: East Sussex, UK. Posts: 389	Re: Mullard High Speed Valve Tester problem Jim - thanks; will do..... Bryan

18th Apr 2007, 9:44 pm	#9
PJL Dekatron Join Date: Sep 2005 Location: Seaford, East Sussex, UK. Posts: 5,997	Re: Mullard High Speed Valve Tester problem I did a full restoration on one of these about a year ago and about 50% of the resistors were out of tolerance (mainly the low tolerance ones). Other problems are the valves as it is easy to leave the power on and hence destroy the valve emissions. Don't test it on low mains settings as there are some quite expensive valves in there. The MAINS card only uses the CRT differential amplifier, the -ve supply and reference voltage so it is best to get this working first. Then move onto the REJECT card and finally the HT card which will fire up the regulated ht supply. The preset A is used to calibrate the MAINS card but you should only adjust this if you are confident that the rest of the tester is properly to tolerance. Accurate mains settings are required for correct heater voltages as the reference and HT are regulated (i.e. using a DVM and setting it manually is fine). I have produced a computer based calculator so if you PM me a card number (preferably a pentode) I can send you the voltages to expect on each pin of the test socket before you go melting a valve...

19th Apr 2007, 12:09 am	#11
brymac Hexode Join Date: Nov 2004 Location: East Sussex, UK. Posts: 389	Re: Mullard High Speed Valve Tester problem Thank you - that is very helpful. I'll have replaced all the capacitors by the weekend and will report progress then! Bryan

20th Apr 2007, 12:40 pm	#13
PJL Dekatron Join Date: Sep 2005 Location: Seaford, East Sussex, UK. Posts: 5,997	Re: Mullard High Speed Valve Tester problem Hi Bryan, I would not recommend following Paul's instructions just yet. This is how I would suggest you go: - Measure your mains with a DMM and set the mains selector accordingly. - With the MAINS card in check the -ve supply (-200V) and regulator (-85V) with a DMM. Don't worry about the spot position as the CRT amp has not been calibrated yet. - With the REJECT card you can check proper operation and calibrate the CRT differential amplifier by following the Mullard instructions. It takes a while getting this right as the controls interact. - With the HT card you can check the HT regulator is working (off load). - With card 68 and the information I have supplied above you can check much of the rest of the circuitry: position 1 - pin 5 or 6 should read -200V; position 2 - pin 5-6=6.3V AC and pressing buttons should apply -126V...; position 6: pin 2=-2V, pin 7=170V... - Once it is all running then calibrate the MAINS card using Pauls instructions

20th Apr 2007, 12:46 pm	#14
brymac Hexode Join Date: Nov 2004 Location: East Sussex, UK. Posts: 389	Re: Mullard High Speed Valve Tester problem Many thanks - I'll try that! The new caps have just arrived, so I'll fit them first. One thing I have noticed is that at the moment the mains fine adjuster makes no difference to the spot position... Bryan

20th Apr 2007, 1:16 pm	#15
PJL Dekatron Join Date: Sep 2005 Location: Seaford, East Sussex, UK. Posts: 5,997	Re: Mullard High Speed Valve Tester problem Could be quite a few things - the negative supply is not working - the matrix contact is not making - the differential amplifier is not working - the function switch needs cleaning...if you get stuck I know the circuit pretty well so just put a message here.